Clamps to retain hoses and the like on conduit or rigid tubing are well known. These clamps generally comprise a flexible band which encircles the hose and a means for adjusting the degree of closure of the band about the hose. Examples of these found in U.S. Pat. No. 2,536,612 issued to Murray, and No. 4,489,464 issued to Massari et al. Other types of clamps include an electrical ground clamp, U.S. Pat. No. 4,623,204 issued to Auclair; and a pipe repair clamp, U.S. Pat. No. 4,705,078 issued to Montgomery.
While adequate for their intended purpose, none of these clamps discloses a means for maintaining a clamp with a consistent diameter and assuring a reproducible closure of the clamp.
In conventional applications, the clamp is installed by tightening the screw with a torque wrench, usually to about 30-38 in. lb. However, complaints of leaks are still received, and a study was conducted on approximately 4000 screws and nuts using a specialized fixture. It was found that using a constant input torque of 30 in. lbs., the force generated by these random screw/nut combinations varied between 100 and 700 pounds. It became apparent that there was considerable variability in the mating of the screw and the nut to the extent that it was impossible to use torque alone in order to obtain reproducibility in terms of the diameter of the clamp and the clamp's ability to secure the hose to the tubing. Another factor is the compressibility of the resilient hose and the force required to close the clamp over the hose. A wide range of force is required depending upon the hose material and the age of the hose. These factors further emphasize that the conventional method of applying a standard torque to tighten the hose clamp is an ineffective means of installing a hose clamp to assure an adequate seal.
Subsequent research into the screw/nut force output problem suggest that it is directly related to the sliding coefficient of friction and the microhardness of the zinc plating itself (zinc is the plating in general use on clamps). Compared to other platings, zinc has a very wide hardness range as shown in the following values:
______________________________________ PLATING TYPE MICROHARDNESS (Kg/sq.mm.) ______________________________________ lead 5-20 cadmium 30-45 zinc 35-125+ iron 140-up ______________________________________
Cadmium and lead have been used for references because they are both reknown for their high degrees of lubricity--lead much more so than cadmium. Also, they both demonstrate a significantly narrower range between limits compared to 90+ units for zinc. Iron has been included to give some perspective to the magnitude of the numbers and, also, because of the known galling and seizing that occurs between unlubricated iron/steel parts. Overall, the data suggests that zinc will at times possess a high degree of lubricity and at other times the lubricity will be significantly reduced. This characteristic trait has been born out by studies pertaining to the amount of force generated by randomly selected screw/nut combinations at constant input torque. Substitution of cadmium for zinc would still introduce a high degree of variability. Thus, it is apparent that torque alone is not an adequate installation procedure.
Another conventional means for installing a clamp is to examine the "gap" between the head of the screw and the nut when the screw is turned in a closing direction. Larger gaps are preferred by assembly line personnel in certain uses with the belief that additional adjustment is available. However, this is more probably an indication that minimal hose compression has been achieved and is a major factor in leak problems. On the other hand, fully bottomed-out clamps (i.e. maximum hose compression due to high force output in which the screw is fully turned in a closing direction) deprive the user of further means of adjustment. An "optimium size" has been considered to be that clamp diameter such that there are 3-4 full turns of the screw remaining before the clamp reaches its fully closed (bottomed-out) diameter.
Thus despite the availability of clamps, there remains a need to provide a clamp having a closure which produces a predictable and consistent diameter and which can be installed easily to assure an effective seal.